The ability to produce a vapour of trappable atoms of a specific atomic species is useful for cold atom apparatus such as those that involve an atomic vapour source being subjected to laser cooling under vacuum. Such apparatus include those where atoms are captured from a background gas, or a beam of atoms, under vacuum.
There are many desirable practical applications that require the use of a source of trappable atomic vapour of specific atoms. For example, a source of atomic vapour of specific atoms is desirable for the production of optical clocks (which use laser cooled atoms) and atom-interferometers (which can be used as gravity sensors or gravity gradient sensors). Additionally, a source of atomic vapours is desirable for experiments with Bose-Einstein-Condensates.
A known method for generating an atomic vapour of trappable atoms, is to use a material which has a sufficient vapour pressure at room temperature, and placing a bulk sample of that material in a vacuum chamber. The supply of atomic vapour is then controlled through the use of a valve between a source and an experimental vacuum chamber. However, this method of generation of atomic vapours cannot be used if the materials which contain the desired atomic species has a negligible vapour pressure at ambient temperature.
A more versatile for generating an atomic vapour of trappable atoms for a greater range of atomic species involves heating a bulk sample of a desired atomic species in an oven or a dispenser, thereby to produce the necessary thermal energy to cause the material to evaporate or sublime into a vacuum chamber. However since ovens intrinsically produce heat, use of ovens with cold atom devices is inherently problematic and may lead to the device being large in size in order to separate the heat source (and consequent background radiation) from parts of the devices where a low temperature is needed. For example with atomic clocks heat can produce associated shifts of the atomic lines and therefore of the clock or frequency output. Consequently optical clocks which use an oven to produce an atomic vapour are relatively large and they also lack fine control.
The known methods for generating atomic vapours, as described above, can be difficult to control, which can prove especially problematic when performing detailed and accurate experiments or processes. In the prior art, in order to address this problem, it is known to achieve higher control when generating atomic vapours in a multi-chamber setup through the use of light induced atomic desorption (LIAD), whereby atoms that have stuck to the inside walls of a vacuum chamber are encouraged to desorb by shining light onto the vacuum chamber walls. In such circumstances, the adsorbed atoms may be sparsely or sporadically distributed, therefore introducing an element of uncertainty into the process, whereby the location and density of atoms may not fulfil the requirements of the application that uses the atomic vapour. However LIAD is only suitable for use with some atomic species and requires intermediate equipment, in addition to the oven or other apparatus used to initially produce an atomic vapour, thereby increasing the size and complexity of the devices.
For some cold atom devices and applications, including optical clocks, atoms for alkaline earth metals such as strontium are desirable. LIAD has not presently been found effective with these atoms. Ovens are conventionally used, causing difficulties with background heat radiation. The difficulty in producing atomic vapours by thermally heating a bulk sample, such as a metal, becomes even more difficult when the material has reacted to form a more stable compound (for example, the melting/boiling point and energy of melting/vaporization is significantly higher for of strontium oxide than for strontium). The temperature required to cause a phase transition in such materials is very high and would result in too much thermal energy being present in a system for processes that require cold atoms.
In addition to applications using cold atoms, a reliable and controllable source of atomic vapour of a specific species can desirable as a thermal source of atoms, whereby the thermal atoms can be used at least in the following exemplary fields: magnetometry (which has application in the field of medical sciences, for example, where thermal atoms might be used to perform experiments such as brain mapping); surface science (using the emitted atoms to coat surfaces); ion physics (for example in Ion Atom collision physics, where one can measure scattering cross sections, charge transfer cross sections etc. in an Ion-Atom collision); bio sciences (exploring the interaction between alkali atoms (Sr, Yb, Mg . . . ) and large bio molecules, including DNA and other molecules, whereby Strontium (Sr) ions, for example, can interact with a bio molecule via sharing/transfer of electron/s to the Sr ion, which might result in a bond, or just charge transfer); chemistry (for example, the formation of molecules including the ultra-cold molecules and the control of a reaction at the quantum level in particular in ultra-cold molecules); and nano technology (for example, to create atomic level structures on a substrate, perhaps in combination with laser cooling techniques).
Atoms can be separated from a bulk sample by “laser ablation” with a laser being directed on bulk samples themselves (as opposed to the adsorbed atoms addressed with LIAD). Laser ablation of this nature is likely to produce too much heat in order to make it a good method for producing trappable atoms for laser cooling. Conventional laser ablation techniques often result in the atoms forming a plasma and so may not be useful for all applications.
A most common mechanism used by laser ablation to separate atoms from the sample is to provide enough energy to locally heat the sample to generate sufficient thermal energy to evaporate or sublimate to form an atomic vapour by heating. Consequently these techniques rely on thermal energy and suffer from at least some of the disadvantages of an oven. An alternative laser ablation technique using femtosecond pulses separates atoms by ionisation, producing high energy free electron that pull the ions out of the sample by electrostatic forces. These femtosecond techniques require very high power pulses and sufficient time gaps between the pulses, affecting controllability and velocity of the atoms in the vapour.
In order to mitigate for at least some of the above problems and disadvantages according to the present invention, there is provided methods and apparatus as claimed in the attached claims.